1. Field of the Invention
The present invention relates in general to automatic feedback control systems for implement positioning means in off-road agricultural and construction industry vehicles, and in particular to automatic control systems for use on tractors for controlling the elevation of an implement attached thereto by connecting means such as a three-point hitch.
2. Description of the Prior Art
Agricultural tractors have traditionally employed hydromechanical draft control systems wherein implement pulling or load forces (i.e., draft) are sensed through either mechanical or electronic sensor means connected to or through an appropriate linkage. Mechanical linkages have inherently limited flexibility and high hysteresis. Electronic control sensors such as load cells also are quite expensive and subject to damage due to overloads, environmental exposure and other problems.
Many different schemes and systems have been developed to automatically control the elevation of an implement attached to an adjustable hitch on a tractor. U.S. patents which teach controlling the elevation of an implement or hitch as a function of measured draft include:
______________________________________ U.S. Pat. No. Inventor ______________________________________ 2,629,306 Rusconi 4,300,638 Katayama et al. 4,301,870 Carre et al. 4,343,365 Rajagopal et al. 4,437,048 Arnold 4,508,176 Wiegardt et al. 4,518,044 Wiegardt et al. ______________________________________
Other U.S. patents disclose schemes and systems for controlling hitch position as a function of wheel slip. A determination of slip is based on a comparison of the speed of one of the driven wheels of the tractor with the true ground speed, as determined by monitoring the speed of an undriven front wheel or by radar means. Such patents include:
______________________________________ U.S. Pat. No. Inventor ______________________________________ 3,834,481 Carlson 4,086,563 Bachman 4,344,499 Van der Lely et al. 4,419,654 Funk 4,485,471 Herwig 4,518,044 Wiegardt et al. ______________________________________
U.S. Pat. No. 3,716,104 to Koenig et al. shows the concept of controlling hitch position as a function of torque on the tractor due to implement load or draft when compared to engine RPM. U.S. Pat. No. 4,465,142 to Van der Lely et al. discloses an alternative control system which compares the desired engine speed, as set by a manual lever, against actual engine speed to obtain a difference signal which is then amplified and used to directly control an electrically activated hydraulic valve that raises or lowers a plow.
Other systems have been developed which monitor actual engine speed. For example, U.S. Pat. No. 4,077,475 to Hino et al. discloses a hitch control system with a "rotary" draft control which monitors both actual engine speed and actual hitch position. In this control mode, two insensitivity threshold sensors are used to create a deadband to prevent hunting and chattering of the solenoid valves. The difference between actual engine speed and desired engine speed is compared against upper and lower threshold values and is used to influence the operation of the hitch positioning control when the engine speed is between predetermined upper and lower speeds. Furthermore, below a yet lower engine speed, the hitch control system automatically raises the hitch to prevent the engine from stalling. The purpose of this mode to keep the working depth of the implement near its desired value, while effectively preventing any accidental engine failure due to overload conditions caused by an engine load which is too heavy for the engine to handle. This patent, however, apparently fails to recognize that the amount of engine lug-down from a predetermined set point or no-load speed can be used as a form of draft load control, since it provides a separate mode, the draft mode, which makes use of a draft force transducer to provide a constant load on the tractor by automatically adjusting the height of the hitch.
It has recently been recognized that completely eliminating mechanical draft sensors and instead using engine speed to determine draft or load force would be beneficial. In D. Rutkowski & J. Welchans, "The Development of An Electronic Draft Control System at Ford Tractor Operations," Proceedings of the National Conference on Fluid Power (held in Detroit, Mich. on Apr. 29-May 1, 1986), pp. 301-306, a microprocessor-based system where actual engine RPM is compared with the expected no-load RPM at a given throttle position is disclosed. This difference, namely the RPM deviation from no-load RPM, is used to calculate draft force by transforming the difference into a draft signal by matching it to the engine's performance, which is represented by a specific point on one of several engine torque curves stored in the system's memory. The system then uses this draft force signal to control a proportional electrohydraulic valve that raises or lowers the hitch. This article shows that such a draft force signal may be used in combination with a hitch position feedback signals, if desired.
In order to operate, then the system must not only be programmed with the specific engine torque curves, but must also know which one of the several torque curves to use, which requires knowing what gear the tractor's transmission is in.
Our work with engine-sensing draft control systems shows that the draft control system described in the aforementioned article has a number of limitations. In particular, a typical tractor is used with a wide variety of implements, some of which tend to dig themselves into the ground, and others of which tend to drive themselves out of the ground. Moreover, the weight of the implements varies dramatically, and this adversely influences the stability of the system. Our experiments show that a number of additional feedback compensation mechanisms, including manual controls for allowing the operator to make field adjustments, are necessary or highly desirable if an engine-sensing draft control system is to be effective for a wide variety of implements. In particular, an engine-sensing draft control system which only allows the operator to adjust the desired load tends to exhibit valve chatter, hunting and instability in a number of situations. Moreover, if the wheels begin to slip, which allows engine RPM to increase, the system mistakenly perceives that draft load is lessening and responds by lowering the hitch, which can result in the implement becoming dug in, thereby stalling the tractor. The system described in the aforementioned article requires the storage of families of engine torque curves, either as tables of values (which can take inordinate amounts of memory in a microprocessor-based control system) or as complex formulas (which can be difficult to program into a micro-processor-based controller and can require significant computation time to convert an engine lug-down value to a draft load force using such formulas). Another problem with using pre-programmed engine torque curve tables or formulas is that they are inaccurate in proportion to the variation of engine performance from ideal conditions. As the engine wears, goes out of tune, or is misadjusted, the accuracy of the conversion from engine lug-down to draft load using such torque curves becomes increasingly inaccurate. Also, knowledge of the gear ratios of each transmission and the actual gear the tractor's transmission is operating in must be obtained, which adds to the cost and complexity of implementing the system.
In light of the foregoing problems, it is an object of the present invention to provide an engine-sensing draft control system which permits the operator to manually adjust a sufficient number of key control system parameters to allow proper operation of the control system with a wide variety of implements of different weights and ground-engagement characteristics. A further object of the invention to provide a control system which is stable in operation and can compensate for the several nonlinearities commonly associated with tractor engine throttles and the hydraulic valve used to operate a three-point hitch, including those introduced by implements of varying weights. Another object is to provide a draft control system which monitors slippage and can avoid excessive slippage. Yet another object of the present invention is to provide an engine-sensing draft control system which does not require knowledge of the engine torque curve characteristics of the vehicle or knowledge of the particular gear in which the vehicle is operating in order to operate effectively.